Composite copper foil, method of production thereof and high frequency transmission circuit using said composite copper foil

ABSTRACT

A composite copper foil excellent in conductivity and surface shape, having high strength and able to be used for applications such as high frequency transmission circuits and a method of production of the same are provided. A composite copper foil characterized by having a copper foil on at least one surface of which a copper and/or silver smoothing layer is provided. Further, producing this by processing an ingot having a copper alloy to a foil having a desired thickness by rolling, then forming on at least one surface of the processed copper alloy foil a smoothing layer by copper plating and/or silver plating. Alternatively, producing this by processing an ingot having a copper alloy to a foil having a thickness of an intermediate size by rolling, forming on at least one surface of the foil a smoothing layer by copper plating and/or silver plating, then rolling the result to a foil having a desired thickness or applying heat treatment or applying heat treatment and rolling to thereby make the thickness of at least the copper and/or silver plating layer at the surface of the foil 0.01 μm or more. Further, a high frequency transmission circuit characterized by being prepared using the above composite copper foil or the composite copper foil produced by the above method of production.

TECHNICAL FIELD

The present invention relates to a composite copper foil excellent instrength, conductivity, and surface shape and a method of production ofthe composite copper foil and for example provides a composite copperfoil optimum for the application of a high frequency transmissioncircuit such as an antenna of an IC card, a method of production of thesame, and a high frequency transmission circuit using the compositecopper foil.

BACKGROUND ART

In recent years, due to the demands for reduction of the size andincrease of the processing speed of high performance electronicequipment, the materials used for their circuit interconnects havegenerally been thin types advantageous for reducing the pitch andlightening the weight and have been required to have a low impedancewith respect to a high frequency current. One example of such equipmentis an IC card.

Up until recently, mainly magnetic strip cards storing magnetic signalshave been widely utilized in various fields such as bank cards, creditcards, telephone cards, and bonus point cards due to their conveniencein carrying. As opposed to this, IC cards have built-in ICs inside thecards, so enable more sophisticated judgment and complex processing.They also have storage capacities about 100 times greater than magneticstrip cards, enable reading/writing of information, and are high insafety.

The methods of transmission of information of IC cards include thecontact type of communicating by physical contact with contacts and alsothe non-contact type enabling communication across a spatial distance ofas much as a few meters using electromagnetic waves etc.

Due to these features of IC cards, IC cards are expected to be utilizedin a very wide range of applications such as ID cards, train and busticket, commuter passes, electronic money, highway passes, healthinsurance cards, resident cards, medical cards, and physicaldistribution control cards.

Non-contact type IC cards are currently classified into four typesaccording to the communication distance: the touch type (communicationdistance up to 2 mm), proximity type (same, up to 10 cm), midrange type(same, up to 70 cm), and microwave type (same, up to several meters).The communication frequencies extend from the MHz to the GHz, forexample, 4.91 MHz in the touch type, 13.56 MHz in the proximity type andthe midrange type, and 2.45 to 5.8 GHz in the microwave type.

A non-contact type IC card basically is constructed from an insulationsheet, an antenna, and an IC chip. The IC chip includes in it aferroelectric memory, nonvolatile memory, ROM, RAM, modem circuit, powersupply circuit, encryption circuit, control circuit, etc. As the antennamember of this IC card, use is made of a covered copper wire coil,silver paste, aluminum foil, copper foil, or the like. These areselectively used according to the number of windings, application,production costs, etc. When the number of windings is small and a highconductivity is necessary, rolled pure copper foil or electrolyticcopper foil is frequently used as the antenna material.

On the other hand, when using foil having large surface roughness suchusual electrolytic copper foil as the material for the antenna, theimpedance increases at the time of transmission and reception of thehigh frequency signal, so sometimes use is not possible in the highfrequency region.

Further, the high strength and high conductivity copper alloy foil nowbeing used as lead frame material etc. has a high material strength whencompared with pure copper foil (hereinafter simply referred to as“copper foil” as opposed to “copper alloy foil”), but is insufficientfor meeting recent demand such as faster speed of signal transmission,reduced size, and higher reliability.

Accordingly, in order to cope with the further reduction of pitch andlightening of weight, various applications for improving thecharacteristics of these conventional copper foil and copper alloy foilhave been filed (refer to for example Japanese Unexamined PatentPublication (Kokai) No. 2002-167633), but none of these satisfy thecharacteristic of reduction of transmission loss in the high frequencyregion as for example an antenna material.

DISCLOSURE OF THE INVENTION

In view of the above recent demands, the inventors engaged in intensiveresearch in order to solve the above problems and as a result succeededin developing a composite copper foil having a high conductivity andalso having a low impedance by providing a layer having a smallresistance like copper and/or silver on its surface and therebyproviding a composite copper foil meeting recent demands. The inventorsprovide a composite copper foil excellent in conductivity and surfaceshape and, by employing a copper alloy rolled foil for applicationswhere strength is particularly demanded, optimum even for applicationsof high frequency transmission circuits such as the antennas of ICcards, and a method of production of the same.

From the viewpoint of the conductivity of the copper or silver layer,the present invention was made based on the idea of, since the currentflows through the surface layer in a high frequency region inapplications of high frequency transmission circuits, arranging copperand/or silver excellent in conductivity at the surface, maintaining thestrength by using copper foil or copper alloy rolled foil (material) asa core material, and, particularly in the case of applications where theusage environment requires repeated bending, employing copper alloyrolled foil excellent in repeated bending strength.

Further, in the present invention, due to arrangement at the surface, ahigh purity is preferable, but it is also possible to add slight amountsof additive elements for alloying.

A first aspect of the invention of the present application provides acomposite copper foil characterized by having a copper foil (including acopper alloy foil) on at least one surface of which a copper and/orsilver smoothing layer is provided.

As the copper foil, preferably a precipitated copper alloy rolled foilis employed.

A thickness of the smoothing layer of copper and/or silver is preferablyat least 0.01 μm or more.

A surface roughness of the smoothing layer is preferably 0.3 to 5.0 μmin terms of Rz and 0.02 to 0.5 μm in terms of Ra.

Preferably, the smoothing layer is treated by either or both rougheningtreatment and/or rust-proofing treatment.

Especially, when strength is required in the usage environment, as thecopper foil, preferably use is made of a copper alloy composite foilhaving a tensile strength of 500 N/mm² or more.

A second aspect of the invention of the present application provides amethod of production of a composite copper foil characterized byprocessing an ingot having a copper alloy to a foil having a desiredthickness by rolling, then forming on at least one surface of theprocessed copper alloy foil a smoothing layer by copper plating and/orsilver plating.

A third aspect of the invention of the present application provides amethod of production of a composite copper foil characterized byprocessing an ingot having a copper alloy to a foil having a thicknessof an intermediate size by rolling, forming on at least one surface ofthe foil a smoothing layer by copper plating and/or silver plating, thenrolling the result to a foil having a desired thickness.

A fourth aspect of the invention of the present application provides amethod of production of a composite copper foil characterized byprocessing an ingot having a copper alloy to a foil having a thicknessof an intermediate size by rolling, forming on at least one surface ofthe foil a smoothing layer by copper plating and/or silver plating, thenapplying heat treatment or applying heat treatment and rolling tothereby make the thickness of at least the copper and/or silver platinglayer at the surface of the foil 0.01 μm or more.

Preferably, a step of treating the smoothing layer of the compositecopper foil produced by the above method of production by rougheningtreatment and/or rust-proofing treatment of the copper is provided.

A fifth aspect of the invention of the present application provides ahigh frequency transmission circuit characterized by being preparedusing the composite copper foil.

BEST MODE FOR WORKING THE INVENTION

The layer of copper and/or silver constituting the smoothing layerformed on the surface of the composite copper foil in the presentinvention is formed on a core material given a desired thickness byplating. The layer of copper and/or silver may also be formed on a corematerial having an intermediate thickness (core material before rolling,annealing, or another process) to obtain an intermediate complex corematerial, then the intermediate complex core material processed to afoil by rolling, annealing, or another process. It is sufficient that inthe end the surface of the foil be left with a thin smoothing layer.

Note that when processing a copper alloy rolled material to anintermediate thickness layer, plating the obtained core material with alayer of copper and/or silver, then heat treating or otherwiseprocessing the result, if the core material given the intermediatethickness is a solid solution type or a precipitated/solid solution typealloy (for example containing zinc etc.), the heat treatment afterplating the layer of copper and/or silver etc. causes the alloyingelement (Zn) to diffuse up to the surface layer (smoothing layer) foralloying up to the surface and therefore there is a risk of lowering theconductivity of the smoothing layer. Accordingly, it is necessary toappropriately set the heat treatment and other conditions and secure theconductivity of the surface layer.

Contrary to this, in the precipitated type, there is little diffusion ofthe alloying element to the surface due to the heating, therefore, thedrop in the conductivity of the surface layer becomes small. This ismore advantageous in comparison with the solid solution type.

When powering up a circuit prepared by conventional copper alloy foilwith a high frequency, the resistance greatly increases due to the skineffect and induces an increase of the impedance, so normaltransmission/reception of signals sometimes becomes impossible. Theinventors analyzed this phenomenon and as a result found that if usingthe conventional copper alloy foil, since the copper alloy foil is lowerin conductivity compared with pure copper foil, the influence of theskin effect is great.

Further, when pure copper foil and copper alloy foil both suffer fromthe above-mentioned trouble when the surface becomes rough. Asindicators of surface roughness, both Rz and Ra are influential.

The inventors conducted various experiments and studies in the presentinvention and as a result found that the copper foil (including copperalloy foil) used as the core material preferably has an Rz of 5.0 μm orless and an Ra of 0.5 μm or less for the skin effect in high frequencytransmission.

On the other hand, if the surface is too smooth, slippage occurs whenconveying the composite copper foil and induces scratches in the foilsurface. In the production and handling of foil (generally “foil” meansfoil with a thickness of 0.080 mm or less), unlike the production andhandling of sheet, the foil must be conveyed on the line with a lowtension due to its thin thickness, the conveyor rolls are harder tosynchronize in comparison with sheet, and therefore slip scratchingeasily occurs. Slip scratching sometimes occurs over the entire lengthof the foil. When strong slip scratching occurs and exceeds 5.0 μm inRz, the foil sometimes forms a fold at this position. Further, a productprocessed using a portion where large slip scratching occurs as acircuit part as it is becomes larger in impedance due to the skin effectin comparison with a product without slip scratching and cannot be usedfor a high frequency transmission circuit.

For this reason, the surface Rz of the composite copper foil ispreferably made 0.3 μm or more, and the Ra is made 0.02 μm or more.

The foil must be high enough in strength to be able to withstand atensile stress etc. acting on it when it is deformed in the process ofassembling parts or when laying interconnects at a narrow pitch.Particularly, in a usage environment where repeated bending etc. isdemanded, the composite copper foil must have a tensile strength of 500N/mm² or more, desirably 700 N/mm² or more. If lower than this, breakageoccurs at the time of assembly work and wrinkles and folding occur whenrolling. This degrades the productivity. In addition, wrinkles areliable to increase the impedance.

In the present invention, by securing the strength of the foil by thecore material (copper foil) and providing a metal having a highconductivity like copper or silver on the surface, the loss due to theskin effect at the time of high frequency transmission is reduced. Therelationship between the frequency and the depth at which the currentflows (skin depth) in a surface layer comprised of silver or copper iscalculated as about 20 μm at 10 MHz, about 3 μm at 0.5 GHz, about 2 μmat 1 GHz, and about 0.6 μm at 10 GHz. Slight roughness of the surface orconductivity (containing impurities) has a large effect.

Regarding the thickness of the copper or silver layer present on thesurface, due also to the added effect of smoothing the surface, athickness of about 1/10 or more of the skin depth corresponding to thefrequency for the application of use is enough to obtain the effect.

That is, in the touch type, proximity type, and midrange type, athickness of about 2 μm is necessary, while in the microwave type, theeffect is exhibited when the thickness is about 0.1 μm.

Note that, for forming a circuit by etching, a copper layer ispreferable over silver since it is easily dissolved away by the sameetchant.

Further, from the high frequency characteristics, the surface ispreferably not formed with a roughened film or a rust-proofing film, butwhen adhesion with a resin etc. and corrosion resistance are required,the high frequency characteristics may be partially sacrificed to formthe roughened film or the rust-proofing film.

For the roughened film, fine particles comprised of Cu or Cu and Co, Ni,Fe, or Cr or a mixture of these and oxides of elements such as V, Mo, orW are electrolytically precipitated. Note that it is preferred tofurther plate the roughened film with Cu to prevent flaking. Normally, adeposition amount of 0.01 mg/dm² or more can improve the adhesion forcewith the substrate resin.

Further, the surface may be further treated for rust-proofing andtreated by a silane coupling agent. For the rust-proofing, generally thesurface is further plated by Ni, Zn, or Cr, or an alloy of the same, istreated by chromate, or is treated for rust-proofing organically by BTA(benzotriazole) etc.

As the silane coupling agent, a vinyl-based one, epoxy-based one, etc.is suitably selected in accordance with the substrate used.

Next, the present invention will be explained in more detail by usingexamples.

Note that this explanation was made for the purpose of giving a generalexplanation of the present invention and has no limitative meaning atall.

EXAMPLE 1

Electric copper was blended in as a main material and a copper berylliummatrix alloy and cobalt as sub materials. These were melted in vacuum ina high frequency melting furnace to produce a copper-beryllium-cobaltalloy. This was cast to an ingot having a thickness of 28 mm.

Next, the ingot was hot processed, repeatedly cold processed andsolution heat treated, then finally cold rolled to obtain foil having athickness of 33 μm. This was then aged. The composition of the obtainedalloy was Be=0.4 wt %, and Co=5.2 wt %.

The surface of the obtained foil was treated by known pre-treatment,then a cyanide bath was used to plate Cu on both surfaces to a thicknessof 1 μm. The surface roughness of the plated composite copper foil was0.2 μm in terms of Ra and 3.1 μm in terms of Rz.

The tensile strength of the obtained composite copper foil was 1010N/mm², and the conductivity was 30 IACS %.

EXAMPLE 2

A copper alloy foil produced in the same way as Example 1 was plated ina cyanide bath with Ag instead of Cu on both surfaces to a thickness of1 μm.

The roughness of the surface was 0.23 μm in terms of Ra and 3.2 μm interms of Rz.

The tensile strength of the obtained copper alloy composite foil was1020 N/mm², and the conductivity was 29 IACS %.

EXAMPLE 3

Electric copper was blended in as a main material and a copper berylliummatrix alloy and cobalt as sub materials. These were melted in vacuum ina high frequency melting furnace in the same formulation as in Example 1to produce a copper-beryllium-cobalt alloy. This was cast to an ingothaving a thickness of 25 mm.

Next, the ingot was hot worked, repeatedly cold worked and solution heattreated, then finally cold rolled to obtain foil having a thickness of29 μm, then the two surfaces were plated with Cu in a copper cyanidebath to a thickness of 3 μm, then aged.

The surface roughness was 0.2 μm in terms of Ra and 2.2 μm in terms ofRz.

The tensile strength of the obtained composite copper foil was 920N/mm², and the conductivity was 36 IACS %.

EXAMPLE 4

The ingot cast in Example 3 was hot worked, repeatedly cold worked andsolution heat treated to obtain a core material having an intermediatethickness of 35 μm, then was plated at both surfaces by copper cyanideto a thickness 3 μm, then finally cold rolled to obtain a compositecopper foil having a thickness of 35 μm. This was then aged.

The roughness of the surface was 0.17 μm in terms of Ra and 2.1 μm interms of Rz.

The tensile strength of the obtained composite foil was 910 N/mm², andthe conductivity was 35 IACS %.

COMPARATIVE EXAMPLE 1

Electric copper was blended in as a main material and a copper berylliummatrix alloy and cobalt as sub materials. These were melted in vacuum ina high frequency melting furnace to produce a copper-beryllium-cobaltalloy. This was cast to an ingot of the same metal composition asExample 1 and having a thickness of 30 mm.

Next, the ingot was hot worked, repeatedly cold worked and solution heattreated, then finally cold rolled to obtain foil having a thickness of35 μm. This was then aged.

The roughness of the surface was 0.3 μm in terms of Ra and 3.6 μm interms of Rz.

The tensile strength was 1080 N/mm², and the conductivity was 26 IACS %.

(Measurement of Transmission Loss (1))

The composite copper foils obtained in Examples 1 to 4 and the copperalloy foil obtained in Comparative Example 1 were measured fortransmission loss.

In the evaluation, each of the copper foils prepared in Examples andComparative Example 1 was placed on a glass fabric prepreg impregnatedwith a high frequency substrate use resin and heat pressed to obtain alaminate, then the foil surface was covered with a dry film etchingresist and etched to prepare a high frequency printed circuit board.Patterns were obtained with a width of the foil of the circuit board of100 μm and a distance between conductors of 100 μm. This was used totransmit a signal of 4 GHz over 500 mm, and the transmission loss wasmeasured.

The rates of reduction of the transmission loss by the examples comparedwith Comparative Example 1 were as follows:

Example 1: 13%

Example 2: 12%

Example 3: 42%

Example 4: 35%

Further, none of the examples suffered from slip scratching etc. inproduction, and the appearances were good.

EXAMPLE 5

8% tin-phosphorus bronze, electric copper, phosphorus-containing copper,and tin were used as starting materials and cast in vacuum to obtain aningot having a thickness of 30 mm. The composition was Sn=8.2 wt % andP=0.03 wt %.

The ingot was hot worked, then repeatedly cold worked and rolled toobtain a foil having a thickness of 30 μm. The obtained foil was treatedby known pretreatment, then a glossy copper sulfate plating bath wasused to plate the two surfaces with copper to a thickness of 2.5 μm.

The surface roughness was 0.2 μm in terms of Ra and 1.8 μm in terms ofRz.

The tensile strength of the obtained composite foil was 610 N/mm², andthe conductivity was 25 IACS %.

EXAMPLE 6

A copper alloy composite foil prepared in the same way as Example 5 was,to simulate low temperature annealing, heated in the atmosphere at 250°C. for 30 minutes, then the surface was pickled by sulfuric acid.

The roughness and the tensile strength were equivalent to those ofExample 5, and the conductivity was 23 IACS %.

EXAMPLE 7

The copper alloy composite foil of Example 5 was burnt plated, thenencapsulated and finely roughening treated. Further, as rust-proofingtreatment, the foil was electroplating with Cr to 0.02 mg/dm² and wastreated by a vinyl-based silane coupling agent.

The roughness was 0.27 μm in terms of Ra and 2.5 μm in terms of Rz, andthe tensile strength and conductivity were equivalent to those ofExample 5.

EXAMPLE 8

The same procedure was followed as in Example 5 to obtain a foil havinga thickness of 34.6 μm. This foil was treated by known pretreatment,then the two surfaces were plated in a cyanide bath with Ag to athickness of 0.1 μm, then were plated with glossy copper sulfate to athickness of 0.1 μm.

The roughness was 0.3 μm in terms of Ra, and 3.0 μm in terms of Rz. Thetensile strength was 692 N/mm², and the conductivity was 13 IACS %.

COMPARATIVE EXAMPLE 2

The ingot having the thickness of 30 mm obtained in Example 5 was hotworked, then repeatedly cold worked and rolled to obtain a foil having athickness of 35 μm.

The surface roughness was 0.4 μm in terms of Ra, and 3.2 μm in terms ofRz. The tensile strength was 700 N/mm², and the conductivity was 12 IACS%.

(Measurement of Transmission Loss (2))

These foils were measured for transmission loss by the same method asthat described above.

The rates of reduction of the transmission loss when comparing Examples5 to 8 and Comparative Example 2 were as follows.

Example 5: 35%

Example 6: 23%

Example 7: 13%

Example 8: 9%

In the above as well, the examples were free from slip scratching inproduction, and the appearances were good.

(Measurement of Strength)

Further, compared with the about 400 N/mm² strength of the conventionalelectrolytic copper foil and pure copper foil obtained by rolling, thecomposite copper foil of the present invention has a high strength ofabout 1000 N/mm² in Examples 1 to 4 and 600 N/mm² or more also inExamples 5 and 8. Further, the repeated bending strength is also aboutthree times as a result of the measurement.

As mentioned above, the composite copper foil of the present inventionhas little high frequency transmission loss in comparison with theconventional electrolytic copper foil and rolling and is excellentparticularly for copper foil for high frequency circuit use.

Further, the present invention is not limited to any special copperalloy and can be applied to both electrolytic copper foil and rolledcopper foil (including alloy foil) having problems particularly for highfrequency transmission circuits due to the surface roughness, so thepresent invention has a high industrial value.

Further, particularly, when using precipitated copper alloy etc., it canbe preferably used for applications where a high strength is required.Its industrial value is therefore high.

Further, the composite copper foil of the present invention is providedwith excellent characteristics as a high frequency transmission circuit,therefore exhibits excellent effects which can be suitably used for anantenna material of contact type and non-contact type IC cards.

Other than them, various modifications are possible within a range notout of the gist of the present invention.

INDUSTRIAL APPLICABILITY

The composite copper foil of the present invention can be applied tocopper foil for a high frequency transmission circuit such as theantenna of an IC card.

The method of production of the composite copper foil of the presentinvention can be applied for producing copper foil for a high frequencytransmission circuit such as the antenna of an IC card.

The high frequency transmission circuit of the present invention can beapplied to the antenna etc. of an IC card.

1. A composite copper foil characterized by comprising a copper foil onat least one surface of which a copper and/or silver smoothing layer isprovided.
 2. A composite copper foil as set forth in claim 1,characterized in that a thickness of said smoothing layer is 0.01 μm ormore.
 3. A composite copper foil as set forth in claim 1, characterizedin that a surface roughness of said smoothing layer is 0.3 to 5.0 μm interms of Rz and 0.02 to 0.5 μm in terms of Ra.
 4. A composite copperfoil as set forth in claim 2, characterized in that a surface roughnessof said smoothing layer is 0.3 to 5.0 μm in terms of Rz and 0.02 to 0.5μm in terms of Ra.
 5. A composite copper foil as set forth in claim 1,characterized in that said copper foil is a copper alloy rolled foil. 6.A composite copper foil as set forth in claim 5, characterized in thatsaid copper alloy rolled foil is a precipitated alloy.
 7. A compositecopper foil as set forth in claim 5, characterized in that a surfaceroughness of said smoothing layer is 0.3 to 5.0 μm in terms of Rz and0.02 to 0.5 μm in terms of Ra.
 8. A composite copper foil as set forthin claim 6, characterized in that a surface roughness of said smoothinglayer is 0.3 to 5.0 μm in terms of Rz and 0.02 to 0.5 μm in terms of Ra.9. A composite copper foil as set forth in any one of claims 1 to 4,characterized in that said smoothing layer is treated by rougheningtreatment and/or rust-proofing treatment.
 10. A composite copper foil asset forth in any one of claims 5 to 8, characterized in that saidsmoothing layer is treated by roughening treatment and/or rust-proofingtreatment.
 11. A composite copper foil as set forth in any one of claims5 to 8, characterized in that a tensile strength of said compositecopper foil including said copper alloy rolled foil is 500 N/mm² ormore.
 12. A composite copper foil as set forth in any one of claim 10,characterized in that a tensile strength of said composite copper foilincluding said copper alloy rolled foil is 500 N/mm² or more.
 13. Amethod of production of a composite copper foil characterized byprocessing an ingot comprising a copper alloy to a foil having a desiredthickness by rolling, then forming on at least one surface of theprocessed copper alloy foil a smoothing layer by copper plating and/orsilver plating.
 14. A method of production of a composite copper foilcharacterized by processing an ingot comprising a copper alloy to a foilhaving a thickness of an intermediate size by rolling, forming on atleast one surface of the foil a smoothing layer by copper plating and/orsilver plating, then rolling the result to a foil having a desiredthickness.
 15. A method of production of a composite copper foilcharacterized by processing an ingot comprising a copper alloy to a foilhaving a thickness of an intermediate size by rolling, forming on atleast one surface of the foil a smoothing layer by copper plating and/orsilver plating, then applying heat treatment or applying heat treatmentand rolling to thereby make the thickness of at least the copper and/orsilver plating layer at the surface of the foil 0.01 μm or more.
 16. Amethod of production of a composite copper foil as set forth in any oneof claims 13 to 15, characterized by further treating said smoothinglayer by roughening treatment and/or rust-proofing treatment of thecopper.
 17. A high frequency transmission circuit characterized by beingprepared using the composite copper foil as set forth in any one ofclaims 1 to
 8. 18. A high frequency transmission circuit characterizedby being prepared using the composite copper foil as set forth in claim9.
 19. A high frequency transmission circuit characterized by beingprepared using the composite copper foil as set forth in claim
 10. 20. Ahigh frequency transmission circuit characterized by being preparedusing the composite copper foil as set forth in claim
 11. 21. A highfrequency transmission circuit characterized by being prepared using thecomposite copper foil as set forth in claim
 12. 22. A high frequencytransmission circuit characterized by being prepared using the compositecopper foil produced by a method of production as set forth in any oneof claims 13 to
 15. 23. A high frequency transmission circuitcharacterized by being prepared using the composite copper foil producedby a method of production as set forth in claim 16.